1. Field
The instant invention relates to processes and systems for removing oxygen from gas streams through the use of a solid state oxygen ion conducting electrolyte. The invention particularly relates to the removal of trace amounts of oxygen as an impurity from inert gas streams.
2. State of the Art
Since the formulation of the Nernst equation in about 1890, it has been known that certain solid metal oxides such as zirconium oxide transported oxygen ions in a relationship to the voltage applied across the electrolyte or, in other words, that a partial pressure differential of oxygen across the electrolyte generated a certain voltage potential. The Nernst equation is stated as follows: ##EQU1##
Since the developments of Ruka and others in the 1960's relating to zirconium oxide, particularly with respect to fuel cells but also with respect to the ability of zirconia to act as an oxygen concentration sensor and transporter of oxygen ions, it has been known that pure oxygen could be delivered by an applied voltage to an electrolyte to separate oxygen from air or other gases. In this regard, see Ruka patent, Re 28,792 (1976).
The technical issues which remain even to the present day is how to bring about an efficient transport of oxygen across an oxygen ion conducting electrolyte, especially when the concentration of oxygen in the inlet gas stream is very low.
The earlier patents and applications of the instant inventors of which this is a continuation-in-part, taught various effective electrode-electrolyte systems for enhancing the transport of oxygen and overcoming some of the innate technical problems associated with such things as proper adherence of electrodes to electrolytes to minimize power loss at the electrode-electrolyte interface, to optimize electrode compositions so that effective current distribution occurred and to select the proper electrodeelectrolyte combinations to optimize electrical efficiency and oxygen ion transport.
In a recent patent, U.S. Pat. No. 5,035,726 assigned to Air Products, a system is described wherein argon containing oxygen as an impurity is pressurized to 30-80 psig and placed in contact with an electrolyte to remove the oxygen. The operation of electrolytes under increased pressure is known from the Nernst equation and there is no data in the Air Products patent to indicate any particularly effectiveness achieved by the stated pressure ranges in comparison with atmospheric pressure operation.
The Air Products patent further notes the use of a sweep gas at the anode side of the gas purifier to reduce the partial pressure of oxygen contained therein. This is done, according to the patent, to reduce the partial pressure differential between oxygen as a trace amount in the inert gas versus the oxygen concentration at the anode side. This, of course, would be expected from knowledge of the Nernst equation. To follow this to its logical conclusion, however, pure sweep gas, that is, containing no oxygen, would have to be swept pass the anode side at the same volumetric rate as the argon on the cathode side in order to create an equivalent partial pressure of oxygen on each side of the electrolyte. Thus, a very large amount of pure sweep gas would generally have to be utilized.
In the Air Products patent, generally the operation was conducted at a voltage of less than about 0.3 volt across the electrolyte. Operation at such low voltages will generally require a great deal of surface area of electrolyte in order to reduce significantly trace amounts of oxygen in an inert gas. Also, at such low voltages, the reductions of the trace amounts to very low levels will not generally be practicable.
Generally, within the inert gas industry it is preferred that oxygen impurities be less than 5 ppm and preferably as low as about 1 ppm. Such low requirements of oxygen in an inert gas means that regardless of whether a sweep gas or other system is used to lower oxygen concentration at the anode side of solid state electrochemical cell, oxygen is being driven from a region of very low concentration at the cathode to a region with a much higher concentration at the anode.